Widespread attention has been focused in recent years on the consequences of bacterial contamination contracted by food consumption or contact with common surfaces and objects. Some noteworthy examples include the sometimes fatal outcome from food poisoning due to the presence of particular strains of Eschericia coli in undercooked beef, Salmonella contamination in undercooked and unwashed poultry food products, and illnesses and skin irritations due to Staphylococcus aureus and other micro-organisms. Anthrax is an acute infectious disease caused by the spore-forming bacterium bacillus anthracis. Allergic reactions to molds and yeasts are a major concern to many consumers and insurance companies alike. Respiratory infections due to viruses such as SARS (severe acute respiratory syndrome) coronavirus, and the return of the H5N1 virus and mutations thereof, now commonly referred to as the avian flu or bird flu, have become major public health issues. In addition, significant fear has arisen in regard to the development of antibiotic-resistant strains of bacteria, such as methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE). The Centers for Disease Control and Prevention estimates that 10% of patients contract additional diseases during their hospital stay and that the total deaths resulting from these nosocomially-contracted illnesses exceeds those suffered from vehicular traffic accidents and homicides. In response to these concerns, manufacturers have begun incorporating antimicrobial agents into materials used to produce objects for commercial, institutional, and residential use.
The antimicrobial properties of silver have been known for several thousand years. The general pharmacological properties of silver are summarized in “Heavy Metals” and “Antiseptics and Disinfectants: Fungicides; Ectoparasiticides,” by Stewart C. Harvey in The Pharmacological Basis of Therapeutics, Fifth Edition, Louis S. Goodman and Alfred Gilman (editors), published by MacMillan Publishing Company, NY, 1975. It is now understood that the affinity of silver ions for biologically important moieties such as sulfhydryl, amino, imidazole, carboxyl, and phosphate groups are primarily responsible for the antimicrobial and antiseptic activity. A detailed review of the oligodynamic effect of silver can be found in “Oligodynamic Metals” by I. B. Romans in Disinfection, Sterlization and Preservation, C. A. Lawrence and S. S. Bloek (editors), published by Lea and Fibiger (1968) and “The Oligodynamic Effect of Silver” by A. Goetz et al. in Silver in Industry, Lawrence Addicks (editor), published by Reinhold Publishing Corporation, 1940. These reviews describe results that demonstrate that silver is effective as an antimicrobial agent capable of destroying or inhibiting the growth of microorganisms, including bacteria, yeast, fungi, and algae, and possibly viruses.
While it is well known that silver-based agents provide excellent antimicrobial properties, aesthetic problems due to discoloration of materials incorporating silver-based agents are frequently a concern. This is believed to be due to several root causes, including the inherent thermal and photo-instability of silver ions, along with other mechanisms. A wide range of silver salts are known to be thermally and photolytically unstable, often discoloring to form brown, gray, or black products. Silver ion may be formally reduced to its metallic state, assuming various physical forms and shapes (particles and filaments), often appearing brown, gray, or black in color. Reduced forms of silver that form particles of sizes on the order of the wavelength of visible light may also appear to be pink, orange, yellow, beige, and other colors due to light scattering effects. Alternatively, silver ion may be formally oxidized to silver peroxide (Ag2O2), a gray-black material, or silver oxide (Ag2O), a brown-black material. In addition, silver ion may simply complex with other materials (e.g. polymer additives, catalyst residues, impurities, surface coatings, etc.) to form colored species without undergoing a formal redox process. Silver ion may attach to various groups on proteins, for example, those present in human skin, resulting in the potentially permanent dark stain condition known as argyria. Silver ion may react with sulfur to form silver sulfide (Ag2S), for which two natural mineral forms, acanthite and argentite, are known, and are black in color. Silver ion may complex with phosphate ion to form Ag3PO4, a yellow material; with chromate ion to form Ag2CrO4, a dark red material; with dichromate ion to form Ag2Cr2O7, also a dark red material; with periodate ion to form AgIO4, an orange-yellow material; or with permanganate ion to form AgMnO4, a dark violet material. While pure silver sulfate is colorless, it has been observed to decompose by exposure to light to a violet color.
In any given situation, a number of mechanisms or root causes may be at work in generating aesthetically displeasing silver-based discoloration, complicating the task of providing a solution to the problem. For example, Coloplast, in U.S. Pat. No. 6,468,521 and U.S. Pat. No. 6,726,791, discloses the development of a stabilized wound dressing having antibacterial, antiviral, and/or antifungal activity characterized in that it comprises silver complexed with a specific amine and is associated with one or more hydrophilic polymers, such that it is stable during radiation sterilization and retains the activity without giving rise to darkening or discoloration of the dressing during storage. Registered as CONTREET®, the dressing product comprises a silver compound complexed specifically with either ethylamine or tri-hydroxymethyl-aminomethane. These specific silver compounds, when used in conjunction with the specific polymer binders carboxymethylcellulose or porcine collagen, are said to have improved resistance to discoloration when exposed to heat, light, or radiation sterilization and contact with skin or tissue.
While the point in time when discoloration of a polymer composite including a silver-based agent appears may range from early in the manufacturing process to late in the useful life of a finished article, we have observed that extreme discoloration is present immediately following melt-processing of a polyamide composite comprising a silver-based agent. While hydrocarbon-based polymeric materials are well known to inherently discolor to some small degree either during high temperature melt processing, or later due to aging in the presence of light, oxygen, and heat, we have observed about a 100-150 fold relative increase in discoloration when a polyamide is melt-processed along with even a modest amount (0.5 weight percent) of a silver-based agent.
Linear aliphatic polyamides, such as nylon 6 or nylon 6,6 polymers, are typically melt-processed at temperatures between about 220-300° C., while partially or wholly aromatic polyamides are typically melt-processed at higher temperatures between about 300-450° C. Relatively small amounts of conventional thermal yellowing of hydrocarbon-based polymers are well understood to occur under these conditions by an oxidative chain reaction process that is initiated by free-radical formation due to the relative weakness of the carbon-hydrogen bond. Free radicals (R*) formed along the hydrocarbon polymer backbone or at terminal positions or on substituents groups resulting from thermally induced homolytic breakage of the carbon-hydrogen bond subsequently react with oxygen (O2) to form peroxy radicals (ROO*), which in turn can react with the polymer to form hydroperoxides (ROOH) and another free radical (R*). The hydroperoxide can then split into two new free radicals, (RO*) and (*OH), which will continue to propagate the reaction at hydrocarbon portions of other polymer chains. It is well known in the art that additives such as antioxidants and light stabilizers can prevent or at least reduce the effects of these oxidative chain reactions. Several types of additives can be added to hydrocarbon-based polymers during processing. Additives are generally divided into groups: stabilizers and modifiers. Typical modifiers include but are not limited to antistatic and antifogging agents, acid scavengers, blowing agents, cling agents, lubricants and resins, nucleating agents, slip- and anti-blocking agents, fillers, flame retardants, compatibilizers, and crosslinkers. Antioxidant stabilizers are typically classified as (1) free-radical scavengers or primary antioxidants, and (2) hydroperoxide decomposers or secondary antioxidants, and serve to counter undesirable consequences (e.g. embrittlement and discoloration) resulting from the thermal instability of the carbon-hydrogen bond. The use of hindered phenols and aromatic amines as antioxidants in hydrocarbon-based polymers is well known in the art.
Approaches in the art to improve the thermal stability of polyamides melt-processed in the absence of a silver-based additive are varied and numerous. French Patent 906,893 and British Pat. 652,947 disclose stabilization of synthetic linear polyamides by the use of copper compounds, while East German Patent 5,350 discloses the use of halogenides, and U.S. Pat. No. 2,510,777 discloses the use of certain acids of phosphorous for this purpose. Mixtures of the above named substances are disclosed in many references, including, for example, in British Patent 722,724 and U.S. Pat. No. 2,705,227; the latter describing a polyamide stabilizer system consisting of a copper compound, a halogen compound, and, optionally, a phosphoric acid or an alkali metal phosphate. While the primary focus of many of these and other references is reduced embrittlement, some also report reduced discoloration. Specifically, British Patent Application 1,140,047 describes a ternary stabilizer system consisting of a copper salt, phosphorous or hypophosphorous acid or a compound of these acids, and an alkali metal halide, which provides molded polyamide compositions with a pale color. Similarly, German Application 2,107,406 discloses a ternary stabilizer system consisting of copper stearate, potassium iodide, and manganese hypophosphite, producing molded polyamide compositions described as colorless. U.S. Pat. No. 2,823,094 discloses that textile materials comprising nylon have a reduced tendency to discolor upon aging or heating when further comprising a compound selected from the group consisting of urea, biuret, dicyandiamide, or ammonium cyanate. U.S. Pat. No. 4,298,518 discloses a polyamide resin composition having excellent appearance and flame-proofing characteristics comprising melamine cyanurate with or without a copper compound, an alkali metal halide, a tin compound, a bisamide compound or a bisureido compound.
Alternatively, many other organic and inorganic compounds have been used for stabilization purposes. It has been suggested to incorporate organic heat stabilizers into the polyamide molecules, for example, amines (Dutch Patent 56,665), mercaptobenzimidazole (U.S. Pat. No. 2,630,421), and n.n-polymethylene-bis-o-hydroxybenzamide (Dutch Patent 55,934). U.S. Pat. No. 5,466,761 discloses nylon alloys with reduced melt-process related yellowing, wherein nylon 4,6 (polytetramethylene adipamide) is mixed with another nylon (e.g. nylon 6 or nylon 6,6) and a nylon copolymer. U.S. Pat. No. 3,282,892 discloses a ternary stabilizer system for polyamides comprising a divalent iron compound, an alkali iodide, and an alkali or earth-alkali phosphate or an organic phosphate having a boiling point above 200° C. U.S. Pat. No. 3,904,705 discloses polyamides stabilized by the incorporation of a mixture of a sterically hindered phenol, a reducing phosphorus compound and a sulphur-containing compound selected from a thiophosphate, a thiodipropionate and a thiocarbamate.
Cairns describes in U.S. Pat. No. 2,430,859 a cross-linked polyamide containing disulfide bonds, prepared through an N-mercaptomethyl polyamide intermediate. This approach to synthesizing a high molecular weight polyamide is extended by Bruck in U.S. Pat. No. 3,299,009 and U.S. Pat. No. 3,331,656, which disclose, in part, a process of asymmetrically and incompletely cross-linking filamentary nylon, said nylon having at least 20% by volume of crystallites, wherein the cross-links comprise disulfide bonds. U.S. Pat. Appl. 20050112339A1 discloses a method for providing antimicrobial protection to plastic structures, such as plastic decking, planking, fencing, and panels, comprising a process of applying a water-soluble biocide (e.g. pyrithione disulfide) to the metal-containing structure, and converting the soluble biocide to a water-insoluble metal biocide salt that is adsorbed on the surface of, or into the porous structure of, the plastic material, to provide slow release of the insoluble antimicrobial agent from the surface or from within the pores of said plastic structure.
U.S. Pat. No. 6,479,144 discloses that spandex fibers prepared by a melt extrusion process to which particles of a silver-based antimicrobial agent (e.g. silver zirconium phosphate, silver glass or silver zeolite) were added along with a standard spandex lubricant (KELMAR® 660), imparted the spandex fibers with excellent anti-tack properties. Uniform distribution as well as a number of non-uniform distributions of the silver-based antimicrobial agent in a sheath/core structured spandex fiber are disclosed.
Extreme discoloration is known to occur in melt-processed thermoplastic polyamides containing a silver-based antimicrobial agent immediately following high temperature compounding, extrusion, or molding. This is described, for example, in US Pat. Appl. 20050183216, which discloses directly incorporating into the melt-processed polyamide (e.g. nylon 6, nylon 6,6, or mixtures thereof) a conventional optical brightener in an amount of about 0.005 to 0.2 percent by weight of the composition, and optionally an antioxidant (e.g. a conventional hindered phenol in combination with an organic phosphite), along with the silver-based antimicrobial agent. The exemplified silver-based antimicrobial agent is a silver-glass, IONPURE™, obtained from the Ishizuka Glass Company. WO2008065110 discloses the addition of ultraviolet light absorbers, or blue light absorbing dyes such as pinacryptol yellow, to textile fibers to act as optical filters to reduce undesired coloring resulting from the photoreduction of a silver-based antimicrobial agent contained therein.
In addition to the color instabilities inherent to silver and silver-based antimicrobial agents, and, to a lesser extent, to the hydrocarbon-based polymeric materials themselves, silver-based agents imbedded in a polymer composite may react with polymer decomposition products, modifiers, stabilizers, and residual addenda (e.g. catalysts) to form potentially colored byproducts. This greater complexity of potential chemical interactions further challenges the modern worker in designing an effective stabilizer for polymers containing silver-based agents.
A number of approaches have been taken in the past to reduce discoloration resulting from the inclusion of silver-based agents in melt-processed polymers. Niira et al in U.S. Pat. No. 4,938,955 disclose melt-processed antimicrobial resin compositions comprising a silver containing zeolite and a single stabilizer (discoloration inhibiting agent) selected from the group consisting of a hindered amine (CHIMASSORB 944LD or TINUVIN 622LD), a benzotriazole, a hydrazine, or a hindered phenol (specifically octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, commercially available as IRGANOX 1076). Reduction in long-term discoloration from exposure to 60 days of sunlight in the air was the only response reported.
Ohsumi et al. in U.S. Pat. No. 5,405,644 disclose two fiber treatment processes in which the addition of a benzotriazole, preferably methylbenzotriazole, to treatment solutions inhibits discoloration in treated fibers comprising a silver-containing tetravalent-metal phosphate antimicrobial agent. More specifically, addition of a benzotriazole to an ester spinning oil was disclosed to reduce discoloration in treated fibers following one day exposure to outdoor sunlight, and the addition of a benzotriazole to an alkali treatment solution reduced discoloration in treated fibers when examined immediately following treatment. Ohsumi et al. hypothesizes that the benzotriazole either retards the dissolution of silver ions or inhibits the reaction of small amounts of soluble silver ion with the various chemicals present in the fiber treatment solutions.
Lever in U.S. Pat. No. 6,187,456 demonstrates reduced thermal yellowing of melt-processed polyolefins, specifically polyamides, containing silver-based antimicrobial agents silver zirconium phosphate or silver zeolite when sodium stearate is replaced with aluminum magnesium hydrotalcite. Tomioka et al. in JP08026921 disclose that discoloration from high temperature can be prevented for polypropylene compounded with a silver-based antimicrobial agent containing specific amounts of sulfite and thiosulfate ion, if the silver-based antimicrobial agent is impregnated on a silica gel support. Dispersing silver-based antimicrobial agents into a wax or low molecular weight polymer as a carrier that is blended into a higher molecular weight polymer is disclosed in JP03271208A and JP28411115B2 as a safe means to handle higher concentrations of silver-based antimicrobial agents without staining the skin.
Reducing discoloration by combining silver-based antimicrobial agents with other antimicrobial agents to reduce the total amount of silver in a given formulation is also known. Ota et al in JP04114038 combine silver sulfate with the organic antifungal agent TBZ (2-(4-thiazolyl)benzimidazole) to reduce discoloration in injection molded polypropylene. Herbst in U.S. Pat. No. 6,585,989 combines a silver containing zeolite and the organic antimicrobial agent TRICLOSAN® (2,4,4′-trichloro-2′-hydroxydiphenyl ether) in polyethylene and polypropylene to yield improved UV stabilization (less yellowness) in accelerated weathering tests.
There is a great need to provide improved melt-processed polyamide composites comprising silver-based antimicrobial agents wherein the degree of aesthetically displeasing discoloration due to the introduction of one or more silver-based antimicrobial agent into the composite or resultant article is substantially reduced.